According to an energy review material published by the NREL, a US National Laboratory, energy sources that are currently used mainly are petroleum, coal and gas. These energy sources amount to 80% of energy sources used in total. However, depletion of petroleum and coal energy has gradually become a big problem currently, and an increase in the emission of carbon dioxide and other greenhouse gases into the air is generating increasingly serious problems. In contrast, the use of renewable energy that is non-polluting green energy is yet about 2% of the total energy source. Thus, concerns about resolving energy source problems have become a motivation for expediting studies on developing new renewable energy. Among the new renewable energy sources such as wind, water and the sun, solar energy has received most attention. Solar cells using solar energy produce less pollution, are limitless in resources and have a semi-permanent lifetime, and therefore, are expected as an energy source capable of resolving future energy problems.
An organic solar cell is a device capable of directly converting solar energy to electric energy by applying a photovoltaic effect. Solar cells are divided into inorganic solar cells and organic solar cells depending on the materials forming a thin film. Typical solar cells are fabricated using a p-n junction by doping crystalline silicon (Si), an inorganic semiconductor. Electrons and holes generated by light absorption spread to p-n junction points, are accelerated by the electric field, and migrate to an electrode. Power conversion efficiency of this process is defined as a ratio of power given to an external circuit and solar power put into a solar cell, and the ratio has been accomplished up to approximately 24% when measured under a currently standardized hypothetical solar irradiation condition. However, existing inorganic solar cells already have limits in economic feasibility and material supplies, and therefore, organic solar cells that are readily processed, inexpensive and have various functions have been highly favored as a long-term alternative energy source.
As for early organic solar cells, the group of Professor Heeger at UCSB of the US has initiatively led the technology development. Monomolecular organic materials or polymer materials used in technologies of a two-step manufacturing process for forming each of a buffer layer and a photoactive layer of an organic solar cell have advantages in that they are readily and quickly used in low-priced and large area processes.
However, a method of manufacturing a buffer layer and photoactive layer in a one-step process through self-phase separation has a problem of not usable in a large-area module solar cell that is not a unit device since constituents of the buffer layer are non-conductors and an ultrathin film needs to be formed. Accordingly, development of technologies capable of being used in a large-area module solar cell while manufacturing a buffer layer and a photoactive layer in a one-step process has been required.